Late autumn tornadoes hit US Midwest

Tornadoes can form across the US in any month of the year, although the peak of the season is usually between March and July. [AFP]

The swarm of tornadoes which swept across the Midwest of the US on Sunday, illustrates just how deadly and devastating late autumn twisters can be.

It is too early to say just how many tornadoes formed during the day but the National Weather Service website listed a possible 81, although there is a chance that some of these reports may have been duplicated. The state of Illionois seems to have been particularly badly hit.

Tornadoes can form across the US in any month of the year, although the peak of the season is usually between March and July.

May and June are the two most active months with around 250 tornadoes being reported countrywide.

By comparison, November sees an average of just 52, but this number exceeds any other month in the July to January period.

The atmosphere is most conducive to tornado formation when the contrast between competing airmasses is at its greatest. In the case of the US, northward moving warm air from the Gulf of Mexico clashes with cold Arctic air driving south across the Plains.

This explains the late spring-early summer peak, but in November the strengthening jet stream, a band of fast-moving air at an altitude of around 10,000 metres, provides the conditions which allow optimum ‘spin’ for a developing storm.

This outbreak was unusual in its northern extent. The state of Illinois seems to have borne the brunt of the tornadic activity. Several twisters of EF4 to EF5 were reported - the highest categories on the Enhanced Fujita scale. This is probably the most significant outbreak this far north to occur so late in the year.

The worst November outbreak on record was in 2002. A wide swathe of the east, from Mississippi and Alabama in the south, to Pennsylvania and Ohio in the north was affected. On 10 and 11 November severe weather was accompanied by 82 tornadoes, 75 of which touched down on the 10th, resulting in the deaths of 36 people.